Sulfinpyrazone, a drug known to inhibit platelet aggregation and prostaglandin synthesis, was found to increase collateral flow to ischemic canine myocardium. Epicardial collateral flow increased 35% five minutes after coronary occlusion and 68% four hours after occlusion as compared to flow during a temporary occlusion before the drug was given. Very small increases in endocardial collateral flow also occurred in the sulfinpyrazone treated animals. The dose of sulfinpyrazone used, 30 mg/kg, was shown to completely inhibit platelet aggregation. The reported beneficial action of sulfinpyrazone in coronary artery disease may be related to drug effects on collateral flow.